Allocating resources and transmitting data in mobile telecommunication systems comprising machine type communication applications

ABSTRACT

A method of communicating data in a wireless telecommunications system between plural base stations and plural terminal devices using plural Orthogonal Frequency Division Multiplex, OFDM, sub-carriers spanning a first frequency bandwidth. The method includes: communicating data between a first base station and a first terminal device using a first OFDM sub-carrier group distributed across the first frequency bandwidth, communicating data between a second base station, geographically separated from the first base station, and a second terminal device using a second OFDM sub-carrier group distributed across a second frequency bandwidth, the second frequency bandwidth being smaller than and within the first frequency bandwidth; and communicating control-plane data between the first base station and the first terminal device using a combination of the first and second groups of the OFDM sub-carriers. Guard regions may be provided in the frequency domain between frequencies of transmissions associated with the first and second base stations.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on PCT Filing PCT/GB2012/051902 filed 6Aug. 2012 and claims priority to British Patent Application 1113800.5,filed in the UK IPO on Aug. 11, 2011, the entire contents of each ofwhich being incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to methods, systems and apparatus forallocating transmission resources and transmitting data in mobiletelecommunication systems.

Third and fourth generation mobile telecommunication systems, such asthose based on the 3GPP defined UMTS and Long Term Evolution (LTE)architecture are able to support more sophisticated services than simplevoice and messaging services offered by previous generations of mobiletelecommunication systems.

For example, with the improved radio interface and enhanced data ratesprovided by LTE systems, a user is able to enjoy high data rateapplications such as mobile video streaming and mobile videoconferencing that would previously only have been available via a fixedline data connection. The demand to deploy third and fourth generationnetworks is therefore strong and the coverage area of these networks,i.e. geographic locations where access to the networks is possible, isexpected to increase rapidly.

The anticipated widespread deployment of third and fourth generationnetworks has led to the parallel development of a class of devices andapplications which, rather than taking advantage of the high data ratesavailable, instead take advantage of the robust radio interface andincreasing ubiquity of the coverage area. Examples include so-calledmachine type communication (MTC) applications, which are typified bysemi-autonomous or autonomous wireless communication devices (i.e. MTCdevices) communicating small amounts of data on a relatively infrequentbasis. Examples include so-called smart meters which, for example, arelocated in a customer's house and periodically transmit information backto a central MTC server data relating to the customers consumption of autility such as gas, water, electricity and so on. Further informationon characteristics of MTC-type devices can be found, for example, in thecorresponding standards, such as ETSI TS 122 368 V10.530 (2011-07)/3GPPTS 22.368 version 10.5.0 Release 10) [1].

Whilst it can be convenient for a terminal such as an MTC type terminalto take advantage of the wide coverage area provided by a third orfourth generation mobile telecommunication network there are at presentdisadvantages. Unlike a conventional third or fourth generation terminaldevice such as a smartphone, an MTC-type terminal is preferablyrelatively simple and inexpensive. The type of functions performed bythe MTC-type terminal (e.g. collecting and reporting back data) do notrequire particularly complex processing to perform. However, third andfourth generation mobile telecommunication networks typically employadvanced data modulation techniques on the radio interface which canrequire more complex and expensive radio transceivers to implement. Itis usually justified to include such complex transceivers in asmartphone as a smartphone will typically require a powerful processorto perform typical smartphone type functions. However, as indicatedabove, there is now a desire to use relatively inexpensive and lesscomplex devices to communicate using LTE type networks. In parallel withthis drive to provide network accessibility to devices having differentoperational functionality, e.g. reduced bandwidth operation, is a desireto optimise the use of the available bandwidth in a telecommunicationssystem supporting such devices.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof operating a base station for communicating data between the basestation and a plurality of terminal devices in a wirelesstelecommunications system using a plurality of Orthogonal FrequencyDivision Multiplex, OFDM, sub-carriers spanning a first frequencybandwidth, the method comprising: communicating user-plane data using afirst group of the OFDM sub-carriers distributed across the firstfrequency bandwidth; communicating no user-plane data using a secondgroup of the OFDM sub-carriers distributed across a second frequencybandwidth, wherein the second frequency bandwidth is smaller than andwithin the first frequency bandwidth; and communicating control-planedata with the plurality of terminal devices using a combination of thefirst and second groups of the OFDM sub-carriers.

In accordance with some embodiments a lower amount of user-plane data iscommunicated using OFDM sub-carriers arranged in frequency at a boundarybetween OFDM sub-carriers in the first group and OFDM sub-carriers inthe second group as compared to an amount of user-plane datacommunicated using OFDM sub-carriers away from this boundary.

In accordance with some embodiments user-plane data is communicated at areduced rate using OFDM sub-carriers at the boundary.

In accordance with some embodiments no user-plane data is communicatedusing OFDM sub-carriers at the boundary.

In accordance with some embodiments, the method further comprisestransmitting reference signals using both the first and second groups ofthe OFDM sub-carriers.

In accordance with some embodiments a density of the reference signalsin the time and/or frequency domain transmitted from the base stationusing the first group of the OFDM sub-carriers is greater than a densityof reference signals in the time and/or frequency domain transmittedfrom the base station using the second group of the OFDM sub-carriers.

In accordance with some embodiments the method further comprisestransmitting reference signals from the base station using only thefirst group of the OFDM sub-carriers at times when control-plane data isnot communicated by the base station.

According to another aspect of the invention there is provided a basestation for communicating data with a plurality of terminal devices in awireless telecommunications system using a plurality of OrthogonalFrequency Division Multiplex, OFDM, sub-carriers spanning a firstfrequency bandwidth, wherein the base station is configured to:communicate user-plane data with the plurality of terminal devices usinga first group of the OFDM sub-carriers distributed across the firstfrequency bandwidth; communicate no user-plane data with the pluralityof terminal devices using a second group of the OFDM sub-carriersdistributed across a second frequency bandwidth, wherein the secondfrequency bandwidth is smaller than and within the first frequencybandwidth; and communicate control-plane data with the plurality ofterminal devices using a combination of the first and second groups ofthe OFDM sub-carriers.

According to another aspect of the invention there is provided a methodof operating a terminal device for communicating data in a wirelesstelecommunications system for communicating data between a plurality ofbase stations and a plurality of terminal devices using a plurality ofOrthogonal Frequency Division Multiplex, OFDM, sub-carriers spanning afirst frequency bandwidth, the wireless telecommunications systemcomprising a first base station configured to communicate using a firstgroup of the OFDM sub-carriers distributed across the first frequencybandwidth; and a second base station configured to communicate using asecond group of the OFDM sub-carriers distributed across a secondfrequency bandwidth, wherein the second frequency bandwidth is smallerthan and within the first frequency bandwidth, and wherein the terminaldevice is configured to communicate only with the second base stationusing the second group of the OFDM sub-carriers and is not operable tocommunicate with the first base station.

According to another aspect of the invention there is provided aterminal device for use in a wireless telecommunications system forcommunicating data between a plurality of base stations and a pluralityof terminal devices using a plurality of Orthogonal Frequency DivisionMultiplex, OFDM, sub-carriers spanning a first frequency bandwidth, thewireless telecommunications system comprising a first base stationconfigured to communicate using a first group of the OFDM sub-carriersdistributed across the first frequency bandwidth; and a second basestation configured to communicate using a second group of the OFDMsub-carriers distributed across a second frequency bandwidth, whereinthe second frequency bandwidth is smaller than and within the firstfrequency bandwidth, and wherein the terminal device is configured tocommunicate only with the second base station using the second group ofthe OFDM sub-carriers and is not operable to communicate with the firstbase station.

According to another aspect of the invention there is provided a methodof communicating data in a wireless telecommunications system between aplurality of base stations and a plurality of terminal devices using aplurality of Orthogonal Frequency Division Multiplex, OFDM, sub-carriersspanning a first frequency bandwidth. The method comprises communicatinguser-plane data between a first base station and a first terminal deviceusing a first group of the OFDM sub-carriers distributed across thefirst frequency bandwidth, communicating user-plane data between asecond base station and a second terminal device using a second group ofthe OFDM sub-carriers distributed across a second frequency bandwidth,wherein the second frequency bandwidth is smaller than and within thefirst frequency bandwidth; and communicating control-plane data betweenthe first base station and the first terminal device using a combinationof the first and second groups of the OFDM sub-carriers.

The second base station may thus use the second group of the OFDMsub-carriers to support communications with associated terminal devicesusing what is in effect a virtual carrier defined within the operatingbandwidth of the first base-station but operating from a different basestation. The virtual carrier may be primarily arranged to serve terminaldevices of a particular kind, for example machine-type terminal devicesassociated with machine-type communications, having reduced operatingcapability compared to terminal devices served by the first basestation. For example, terminal devices served by the second base stationmay have limited operating bandwidth capability commensurate with thesecond frequency bandwidth.

The second base station may be what might be termed a micro-, pico-,femto- or home base station (e-NodeB). Furthermore, in some examples themethod may further comprise communicating relay data between the firstbase station and the second base station using the first group of theOFDM sub-carriers or using a wired link, for example via an internetconnection.

In some examples a reduced amount of user-plane data may be communicatedusing OFDM sub-carriers arranged in frequency at a boundary between OFDMsub-carriers in the first group and OFDM sub-carriers in the secondgroup as compared to the amount of user-plane data communicated usingOFDM sub-carriers away from the boundary. For example, user-plane datamight be communicated at a lower rate using the OFDM sub-carriers at theboundary, or no user-plane data might be communicated using the OFDMsub-carriers at the boundary. Providing guard regions such as this canhelp reduce interference from unintended overlaps in transmissions fromthe two base stations if there is a frequency mismatch between them, forexample.

The method may further comprise transmitting reference signals from thefirst base station using both the first and second groups of the OFDMsub-carriers. This can assist conventional mobile terminals camped-on tothe first (host) base station operate in their normal manner regardlessof the split in transmission resources from within a singletime-frequency grid between different base stations.

A density of reference signals in time and/or frequency from the firstbase station using the first group of the OFDM sub-carriers may begreater than a density of reference symbols transmitted from the firstbase station using the second group of the OFDM sub-carriers. This canprovide a compromise between disruption to the operation of mobileterminals camped-on to the first base station which expect to seereference symbols across the full operating bandwidth of the first basestation, and disruption to the available resources for allocation by thesecond base station using the second group of sub-carriers.

A reduced amount of user-plane data may be communicated using at leastone OFDM sub-carrier which is adjacent in frequency to an OFDMsub-carrier on which a reference signal is transmitted in the secondgroup of the OFDM sub-carriers as compared to an amount of user-planedata communicated using an OFDM sub-carrier in the second group that isnot adjacent in frequency to the reference signal. For example, a lowerrate of data, or no data, may be transmitted here. This use of guardregions can again help reduce interference from unintended overlaps intransmissions from the two base stations should there be a frequencymismatch between them.

In another example, reference signals from the first base station mightbe transmitted using only the first group of the OFDM sub-carriers whilethe second base station is transmitting user-plane date.

In some example implementations there may be no user-plane datacommunicated between the second base station and the second terminaldevice while control-plane data is being communicated between the firstbase station and the first terminal device.

The method may further comprise communicating user-plane data between afurther base station and a further terminal device using the secondgroup of the OFDM sub-carriers. Thus the second and further basestations may be configured to communicate data using the same resources,thereby providing for efficient re-use of the available resources. Aswith the second base station, the further base station may, in someexamples, act as a relay base stations for the first base station andcommunicating relay data with first base station using the first groupof the OFDM sub-carriers or using a wired link, for example via aninternet connection.

A geographic extent of a communication cell associated with the secondbase station may be smaller than and within a geographic extent of acommunication cell associated with the first base station.

According to another aspect of the invention there is provided awireless telecommunications system for communicating data between aplurality of base stations and a plurality of terminal devices using aplurality of Orthogonal Frequency Division Multiplex, OFDM, sub-carriersspanning a first frequency bandwidth, the system comprising: a firstbase station configured to communicate user-plane data with a firstterminal device using a first group of the OFDM sub-carriers distributedacross the first frequency bandwidth; and a second base stationconfigured to communicate user-plane data with a second terminal deviceusing a second group of the OFDM sub-carriers distributed across asecond frequency bandwidth, wherein the second frequency bandwidth issmaller than and within the first frequency bandwidth, wherein the firstbase station is configured to communicate control-plane data with thefirst terminal device using a combination of the first and second groupsof the OFDM sub-carriers.

It will be appreciated that features and aspects of the inventiondescribed above in relation to the first and other aspects of theinvention are equally applicable and may be combined with embodiments ofthe invention according to the different aspects of the invention asappropriate, and not just in the specific combinations described above.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings where likeparts are provided with corresponding reference numerals and in which:

FIG. 1 provides a schematic diagram illustrating an example of aconventional mobile telecommunication network;

FIG. 2 provides a schematic diagram illustrating a conventional LTEradio frame;

FIG. 3 provides a schematic diagram illustrating an example of aconventional LTE downlink radio sub-frame;

FIG. 4 provides a schematic diagram illustrating a conventional LTE“camp-on” procedure;

FIG. 5 provides a schematic diagram illustrating an LTE downlink radiosub-frame in which a virtual carrier has been inserted in accordancewith an embodiment of the invention;

FIG. 6 provides a schematic diagram illustrating an adapted LTE“camp-on” procedure for camping on to a virtual carrier;

FIG. 7 provides a schematic diagram illustrating LTE downlink radiosub-frames in accordance with an embodiment of the present invention;

FIG. 8 provides a schematic diagram illustrating a physical broadcastchannel (PBCH);

FIG. 9 provides a schematic diagram illustrating an LTE downlink radiosub-frame in accordance with an embodiment of the present invention;

FIG. 10 provides a schematic diagram illustrating an LTE downlink radiosub-frame in which a virtual carrier has been inserted in accordancewith an embodiment of the invention;

FIGS. 11A to 11D provide schematic diagrams illustrating positioning oflocation signals within a LTE downlink sub-frame according toembodiments of the present invention;

FIG. 12 provides a schematic diagram illustrating a group of sub-framesin which two virtual carriers change location within a host carrier bandaccording to an embodiment of the present invention;

FIGS. 13A to 13C provide schematic diagrams illustrating LTE uplinksub-frames in which an uplink virtual carrier has been inserted inaccordance with an embodiment of the present invention;

FIG. 14 provides a schematic diagram showing part of an adapted LTEmobile telecommunication network arranged in accordance with an exampleof the present invention;

FIG. 15A schematically represents an example allocation of transmissionresources between a host and virtual carrier in a LTE mobiletelecommunication network arranged according to an embodiment of theinvention;

FIG. 15B schematically represents an example allocation of transmissionresources for a host carrier in a LTE mobile telecommunication networkarranged according to an embodiment of the invention;

FIG. 15C schematically represents an example allocation of transmissionresources for a virtual carrier in a LTE mobile telecommunicationnetwork arranged according to an embodiment of the invention;

FIG. 16 schematically shows a mobile telecommunication networkarchitecture according to an embodiment of the invention;

FIGS. 17A and 17B schematically represents two example allocations oftransmission resources for a host carrier in a LTE mobiletelecommunication network arranged according to an embodiment of theinvention;

FIG. 18 schematically represents an example allocation of transmissionresources between a host and virtual carrier in a LTE mobiletelecommunication network arranged according to an embodiment of theinvention; and

FIG. 19 schematically represents an example allocation of transmissionresources for a virtual carrier in a LTE mobile telecommunicationnetwork arranged according to an embodiment of the invention.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Embodiments of the invention may in particular be employed within thecontext of what might be termed “virtual carriers” operating within abandwidth of a “host carriers”. The concepts of virtual carriers aredescribed in co-pending UK patent applications numbered GB 1101970.0[2], GB 1101981.7[3], GB 1101966.8[4], GB 1101983.3[5], GB 1101853.8[6],GB 1101982.5 [7], GB 1101980.9[8] and GB 1101972.6 [9], the contents ofwhich are incorporated herein by reference. The reader is referred tothese co-pending applications for more details, but for ease ofreference an overview of the concept of virtual carriers is alsoprovided here.

Conventional Network

FIG. 1 provides a schematic diagram illustrating some basicfunctionality of a conventional mobile telecommunications network.

The network includes a plurality of base stations 101 connected to acore network 102. Each base station provides a coverage area 103 (i.e. acell) within which data can be communicated to and from terminal devices104. Data is transmitted from base stations 101 to terminal devices 104within their respective coverage areas 103 via a radio downlink. Data istransmitted from terminal devices 104 to the base stations 101 via aradio uplink. The core network 102 routes data to and from the terminaldevices 104 via the respective base stations 101 and provides functionssuch as authentication, mobility management, charging and so on.

Mobile telecommunications systems such as those arranged in accordancewith the 3GPP defined Long Term Evolution (LTE) architecture use anorthogonal frequency division multiplex (OFDM) based interface for theradio downlink (so-called OFDMA) and the radio uplink (so-calledSC-FDMA). FIG. 2 shows a schematic diagram illustrating an OFDM basedLTE downlink radio frame 201. The LTE downlink radio frame istransmitted from an LTE base station (known as an enhanced Node B) andlasts 10 ms. The downlink radio frame comprises ten sub-frames, eachsub-frame lasting 1 ms. A primary synchronisation signal (PSS) and asecondary synchronisation signal (SSS) are transmitted in the first andsixth sub-frames of the LTE frame. A primary broadcast channel (PBCH) istransmitted in the first sub-frame of the LTE frame. The PSS, SSS andPBCH are discussed in more detail below.

FIG. 3 is a schematic diagram of a grid which illustrates the structureof an example conventional downlink LTE sub-frame. The sub-framecomprises a predetermined number of symbols which are transmitted over a1 ms period. Each symbol comprises a predetermined number of orthogonalsub-carriers distributed across the bandwidth of the downlink radiocarrier.

The example sub-frame shown in FIG. 3 comprises 14 symbols and 1200sub-carriers spread across a 20 MHz bandwidth. The smallest allocationof user data for transmission in LTE is a resource block comprisingtwelve sub-carriers transmitted over one slot (0.5 sub-frame). Forclarity, in FIG. 3, each individual resource element is not shown,instead each individual box in the sub-frame grid corresponds to twelvesub-carriers transmitted on one symbol.

FIG. 3 shows in hatching resource allocations for four LTE terminals340, 341, 342, 343. For example, the resource allocation 342 for a firstLTE terminal (UE 1) extends over five blocks of twelve sub-carriers(i.e. 60 sub-carriers), the resource allocation 343 for a second LTEterminal (UE2) extends over six blocks of twelve sub-carriers and so on.

Control channel data is transmitted in a control region 300 (indicatedby dotted-shading in FIG. 3) of the sub-frame comprising the first nsymbols of the sub-frame where n can vary between one and three symbolsfor channel bandwidths of 3 MHz or greater and where n can vary betweentwo and four symbols for channel bandwidths of 1.4 MHz. For the sake ofproviding a concrete example, the following description relates to hostcarriers with a channel bandwidth of 3 MHz or greater so the maximumvalue of n will be 3. The data transmitted in the control region 300includes data transmitted on the physical downlink control channel(PDCCH), the physical control format indicator channel (PCFICH) and thephysical HARQ indicator channel (PHICH).

PDCCH contains control data indicating which sub-carriers on whichsymbols of the sub-frame have been allocated to specific LTE terminals.Thus, the PDCCH data transmitted in the control region 300 of thesub-frame shown in FIG. 3 would indicate that UE1 has been allocated theblock of resources identified by reference numeral 342, that UE2 hasbeen allocated the block of resources identified by reference numeral343, and so on.

PCFICH contains control data indicating the size of the control region(i.e. between one and three symbols).

PHICH contains HARQ (Hybrid Automatic Request) data indicating whetheror not previously transmitted uplink data has been successfully receivedby the network.

Symbols in a central band 310 of the time-frequency resource grid areused for the transmission of information including the primarysynchronisation signal (PSS), the secondary synchronisation signal (SSS)and the physical broadcast channel (PBCH). This central band 310 istypically 72 sub-carriers wide (corresponding to a transmissionbandwidth of 1.08 MHz). The PSS and SSS are synchronisation signals thatonce detected allow an LTE terminal device to achieve framesynchronisation and determine the cell identity of the enhanced Node Btransmitting the downlink signal. The PBCH carries information about thecell, comprising a master information block (MIB) that includesparameters that LTE terminals use to properly access the cell. Datatransmitted to individual LTE terminals on the physical downlink sharedchannel (PDSCH) can be transmitted in other resource elements of thesub-frame. Further explanation of these channels is provided below.

FIG. 3 also shows a region of PDSCH containing system information andextending over a bandwidth of R₃₄₄. A conventional LTE frame will alsoinclude reference signals which are discussed further below but notshown in FIG. 3 in the interests of clarity.

The number of sub-carriers in an LTE channel can vary depending on theconfiguration of the transmission network. Typically this variation isfrom 72 sub carriers contained within a 1.4 MHz channel bandwidth to1200 sub-carriers contained within a 20 MHz channel bandwidth (asschematically shown in FIG. 3). As is known in the art, data transmittedon the PDCCH, PCFICH and PHICH is typically distributed on thesub-carriers across the entire bandwidth of the sub-frame to provide forfrequency diversity. Therefore a conventional LTE terminal must be ableto receive the entire channel bandwidth in order to receive and decodethe control region.

FIG. 4 illustrates an LTE “camp-on” process, that is, the processfollowed by a terminal so that it can decode downlink transmissionswhich are sent by a base station via a downlink channel. Using thisprocess, the terminal can identify the parts of the transmissions thatinclude system information for the cell and thus decode configurationinformation for the cell.

As can be seen in FIG. 4, in a conventional LTE camp-on procedure, theterminal first synchronizes with the base station (step 400) using thePSS and SSS in the centre band and then decodes the PBCH (step 401).Once the terminal has performed steps 400 and 401, it is synchronizedwith the base station.

For each sub-frame, the terminal then decodes the PCFICH which isdistributed across the entire bandwidth of carrier 320 (step 402). Asdiscussed above, an LTE downlink carrier can be up to 20 MHz wide (1200sub-carriers) and an LTE terminal therefore has to have the capabilityto receive and decode transmissions on a 20 MHz bandwidth in order todecode the PCFICH. At the PCFICH decoding stage, with a 20 MHz carrierband, the terminal operates at a much larger bandwidth (bandwidth ofR₃₂₀) than during steps 400 and 401 (bandwidth of R₃₁₀) relating tosynchronization and PBCH decoding.

The terminal then ascertains the PHICH locations (step 403) and decodesthe PDCCH (step 404), in particular for identifying system informationtransmissions and for identifying its personal allocation grants. Theallocation grants are used by the terminal to locate system informationand to locate its data in the PDSCH. Both system information andpersonal allocations are transmitted on PDSCH and scheduled within thecarrier band 320. Steps 403 and 404 also require the terminal to operateon the entire bandwidth R320 of the carrier band.

At steps 402 to 404, the terminal decodes information contained in thecontrol region 300 of a sub-frame. As explained above, in LTE, the threecontrol channels mentioned above (PCFICH, PHICH and PDCCH) can be foundacross the control region 300 of the carrier where the control regionextends over the range R₃₂₀ and occupies the first one, two or threeOFDM symbols of each sub-frame as discussed above. In a sub-frame,typically the control channels do not use all the resource elementswithin the control region 300, but they are scattered across the entireregion, such that a LTE terminal has to be able to simultaneouslyreceive the entire control region 300 for decoding each of the threecontrol channels.

The terminal can then decode the PDSCH (step 405) which contains systeminformation or data transmitted for this terminal.

As explained above, in an LTE sub-frame the PDSCH generally occupiesgroups of resource elements which are neither in the control region norin the resource elements occupied by PSS, SSS or PBCH. The data in theblocks of resource elements 340, 341, 342, 343 allocated to thedifferent mobile communication terminals (UEs) shown in FIG. 3 have asmaller bandwidth than the bandwidth of the entire carrier, although todecode these blocks a terminal first receives the PDCCH spread acrossthe frequency range R₃₂₀ to determine if the PDCCH indicates that aPDSCH resource is allocated to the UE and should be decoded. Once a UEhas received the entire sub-frame, it can then decode the PDSCH in therelevant frequency range (if any) indicated by the PDCCH. So forexample, UE 1 discussed above decodes the whole control region 300 andthen the data in the resource block 342.

Virtual Downlink Carrier

Certain classes of devices, such as MTC devices (e.g. semi-autonomous orautonomous wireless communication devices such as smart meters asdiscussed above), support communication applications that arecharacterised by the transmission of small amounts of data at relativelyinfrequent intervals and can thus be considerably less complex thanconventional LTE terminals. In many scenarios, providing low capabilityterminals such as those with a conventional high-performance LTEreceiver unit capable of receiving and processing data from an LTEdownlink frame across the full carrier bandwidth can be overly complexfor a device which only needs to communicate small amounts of data. Thismay therefore limit the practicality of a widespread deployment of lowcapability MTC type devices in an LTE network. It is preferable insteadto provide low capability terminals such as MTC devices with a simplerreceiver unit which is more proportionate with the amount of data likelyto be transmitted to the terminal. As set out below, in accordance withexamples of the present invention a “virtual carrier” is provided withinthe transmission resources of a conventional OFDM type downlink carrier(i.e. a “host carrier”). Unlike data transmitted on a conventional OFDMtype downlink carrier, data transmitted on the virtual carrier can bereceived and decoded without needing to process the full bandwidth ofthe downlink host OFDM carrier. Accordingly, data transmitted on thevirtual carrier can be received and decoded using a reduced complexityreceiver unit.

FIG. 5 provides a schematic diagram illustrating an LTE downlinksub-frame which includes a virtual carrier inserted in a host carrier inaccordance with an example of the present invention.

In keeping with a conventional LTE downlink sub-frame, the first nsymbols (n is three in FIG. 5) form the control region 300 which isreserved for the transmission of downlink control data such as datatransmitted on the PDCCH. However, as can be seen from FIG. 5, outsideof the control region 300 the LTE downlink sub-frame includes a group ofresource elements positioned in this example below the central band 310which form a virtual carrier 501. As explained further below, thevirtual carrier 501 is adapted so that data transmitted on the virtualcarrier 501 can be treated as logically distinct from data transmittedin the remaining parts of the host carrier and can be decoded withoutdecoding all the control data from the control region 300. Although FIG.5 shows the virtual carrier occupying frequency resources below thecentre band, in general the virtual carrier can occupy other frequencyresources, for example, above the centre band or including the centreband. If the virtual carrier is configured to overlap any resources usedby the PSS, SSS or PBCH of the host carrier, or any other signaltransmitted by the host carrier that a terminal device operating on thehost carrier would require for correct operation and expect to find in aknown pre-determined location, the signals on the virtual carrier can bearranged such that these aspects of the host carrier signal aremaintained.

As can be seen from FIG. 5, data transmitted on the virtual carrier 501is transmitted across a limited bandwidth. This might be any suitablebandwidth smaller than that of the host carrier. In the example shown inFIG. 5 the virtual carrier is transmitted across a bandwidth comprising12 blocks of 12 sub-carriers (i.e. 144 sub-carriers), which isequivalent to a 2.16 MHz transmission bandwidth. Accordingly, a terminalusing the virtual carrier need only be equipped with a receiver capableof receiving and processing data transmitted over a bandwidth of 2.16MHz. This enables low capability terminals (for example MTC typeterminals) to be provided with simplified receiver units yet still beable to operate within an OFDM type communication network which, asexplained above, conventionally requires terminals to be equipped withreceivers capable of receiving and processing an OFDM signal across theentire bandwidth of the signal.

As explained above, in OFDM-based mobile communication systems such asLTE, downlink data is dynamically assigned to be transmitted ondifferent sub-carriers on a sub-frame by sub-frame basis. Accordingly,in every sub-frame the network signals which sub-carriers on whichsymbols contain data relevant to which terminals (i.e. downlink grantsignalling).

As can be seen from FIG. 3, in a conventional downlink LTE sub-framethis information is transmitted on the PDCCH during the first symbol orsymbols of the sub-frame. However, as previously explained, theinformation transmitted in the PDCCH is spread across the entirebandwidth of the sub-frame and therefore cannot be received by a mobilecommunication terminal with a simplified receiver unit capable only ofreceiving the reduced bandwidth virtual carrier.

Accordingly, as can be seen in FIG. 5, the final symbols of the virtualcarrier can be reserved as a control region 502 for the virtual carrierfor the transmission of control data indicating which resource elementsof the virtual carrier 501 have been allocated to user equipment (UEs)using the virtual carrier. In some examples the number of symbolscomprising the virtual carrier control region 502 might be fixed, forexample three symbols. In other examples the virtual carrier controlregion 502 can vary in size, for example between one and three symbols,as with the control region 300.

The virtual carrier control region can be located at any suitableposition, for example in the first few symbols of the virtual carrier.In the example of FIG. 5 this could mean positioning the virtual carriercontrol region on the fourth, fifth and sixth symbols. However, fixingthe position of the virtual carrier control region in the final symbolsof the sub-frame can be useful because the position of the virtualcarrier control region will not vary in dependence on the number ofsymbols of the host carrier control region 300. This can help simplifythe processing undertaken by mobile communication terminals receivingdata on the virtual carrier because there is no need for terminals todetermine a position of the virtual carrier control region everysub-frame if it is known that it will always be positioned in the finaln symbols of the sub-frame.

In a further embodiment, the virtual carrier control symbols mayreference virtual carrier PDSCH transmissions in a separate sub-frame.

In some examples the virtual carrier may be located within the centreband 310 of the downlink sub-frame. This can help reduce the impact onhost carrier PDSCH resources caused by the introduction of the virtualcarrier within the host carrier bandwidth since the resources occupiedby the PSS/SSS and PBCH would be contained within the virtual carrierregion and not the remaining host carrier PDSCH region. Therefore,depending on for example the expected virtual carrier throughput, thelocation of a virtual carrier can be appropriately chosen to eitherexist inside or outside the centre band according to whether the host orvirtual carrier is chosen to bear the overhead of the PSS, SSS and PBCH.

Virtual Carrier “Camp-On” Process

As explained above, before a conventional LTE terminal can begintransmitting and receiving data in a cell, it first camps on to thecell. An adapted camp-on process can be provided for terminals using thevirtual carrier.

FIG. 6 shows a flow diagram schematically illustrating a camp-on processaccording to an example of the present invention. There are two branchesshown in FIG. 6. Different steps of the process associated with a UEintending to use the virtual carrier are shown under the general heading“virtual carrier”. The steps shown under the general heading “legacyLTE” are associated with a UE intending to use the host carrier, andthese steps correspond to the steps of FIG. 4. In this example, thefirst two steps 400, 401 of the camp-on procedure are common to both thevirtual carrier and host (legacy LTE) carrier.

The virtual carrier camp-on process is explained with reference to theexample sub-frame shown in FIG. 5 in which a virtual carrier with abandwidth of 144 sub-carriers is inserted within the operating bandwidthof a host carrier with a bandwidth corresponding to 1200 sub-carriers.As discussed above, a terminal having a receiver unit with anoperational bandwidth of less than that of the host carrier cannot fullydecode data in the control region of sub-frames of the host carrier.However, a receiver unit of a terminal having an operational bandwidthof only twelve blocks of twelve sub-carriers (i.e. 2.16 MHz) can receivecontrol and user data transmitted on this example virtual carrier 502.

As noted above, in the example of FIG. 6, the first steps 400 and 401for a virtual carrier terminal are the same as the conventional camp-onprocess shown in FIG. 4, although a virtual carrier terminal may extractadditional information from the MIB as described below. Both types ofterminals (i.e. virtual carrier terminals and host/legacy carrierterminals) can use the PSS/SSS and PBCH to synchronize with the basestation using the information carried on the 72 sub-carrier centre bandwithin the host carrier. However, where the conventional LTE terminalsthen continue with the process by performing the PCFICH decoding step402, which requires a receiver unit capable of receiving and decodingthe host carrier control region 300, a terminal camping on to the cellto receive data on the virtual carrier (which may be referred to as a“virtual carrier terminal”) performs steps 606 and 607 instead.

In a further embodiment of the present invention a separatesynchronisation and PBCH functionality can be provided for the virtualcarrier device as opposed to re-using the same conventional initialcamp-on processes of steps 400 and 401 of the host carrier device.

At step 606, the virtual carrier terminal locates a virtual carrier, ifany is provided within the host carrier, using a virtualcarrier-specific step. Various examples of how this step may beperformed are discussed further below. Once the virtual carrier terminalhas located a virtual carrier, it can access information within thevirtual carrier. For example, if the virtual carrier mirrors theconventional LTE resource allocation method, the virtual carrierterminal may proceed to decode control portions within the virtualcarrier, which can, for example, indicate which resource elements withinthe virtual carrier have been allocated for a specific virtual carrierterminal or for system information. For example, FIG. 7 shows the blocksof resource elements 350 to 352 within virtual carrier 330 that havebeen allocated for the sub-frame SF2. However, there is no requirementfor the virtual carrier terminal to follow or mirror the conventionalLTE process (e.g. steps 402-404) and these steps may for example beimplemented very differently for a virtual carrier camp-on process.

Regardless of the virtual carrier terminal following a LTE-like step ora different type of step when performing step 607, the virtual carrierterminal can then decode the allocated resource elements at step 608 andthereby receive data transmitted by the base station broadcasting thevirtual carrier. The data decoded in step 608 may include, for example,the remainder of the system information containing details of thenetwork configuration.

Even though the virtual carrier terminal does not have the bandwidthcapabilities to decode and receive downlink data if it was transmittedin the host carrier using conventional LTE, it can still access avirtual carrier within the host carrier having a limited bandwidthwhilst re-using the initial LTE steps. Step 608 may also be implementedin a LTE-like manner or in a different manner. For example, multiplevirtual carrier terminals may share a virtual carrier and have grantsallocated to manage the virtual carrier sharing as shown in SF2 in FIG.7, or, in another example, a virtual carrier terminal may have theentire virtual carrier allocated for its own downlink transmissions, orthe virtual carrier may be entirely allocated to a virtual carrierterminal for a certain number of sub-frame only, etc.

There is thus a large degree of flexibility provided for the virtualcarrier camp-on process. There is, for example, the ability to adjust abalance between re-using or mirroring conventional LTE steps orprocesses, thereby reducing the terminal complexity and the need toimplement new elements, and adding new virtual carrier specific aspectsor implementations, thereby potentially optimizing the use ofnarrow-band virtual carriers, as LTE has been designed with thelarger-band host carriers in mind.

Downlink Virtual Carrier Detection

As discussed above, the virtual carrier terminal should locate (withinthe time-frequency resource grid of the host carrier) the virtualcarrier before it can receive and decode transmissions on the virtualcarrier. Several alternatives are available for the virtual carrierpresence and location determination, which can be implemented separatelyor in combination. Some of these options are discussed below.

To facilitate the virtual carrier detection, the virtual carrierlocation information may be provided to the virtual carrier terminalsuch that it can locate the virtual carrier, if any exists, more easily.For example, such location information may comprise an indication thatone or more virtual carriers are provided within the host carrier, orthat the host carrier does not currently provide any virtual carrier. Itmay also comprise an indication of the virtual carrier's bandwidth, forexample in MHz or blocks of resource elements. Alternatively, or incombination, the virtual carrier location information may comprise thevirtual carrier's centre frequency and bandwidth, thereby giving thevirtual carrier terminal the location and bandwidth of any activevirtual carrier. In the event the virtual carrier is to be found at adifferent frequency position in each sub-frame, according, for example,to a pseudo-random hopping algorithm, the location information can, forexample, indicate a pseudo random parameter. Such parameters may includea starting frame and parameters used for the pseudo-random algorithm.Using these pseudo-random parameters, the virtual carrier terminal canthen know where the virtual carrier can be found for any sub-frame.

On implementation feature associated with little change to the virtualcarrier terminal (as compared with a conventional LTE terminal) would beto include location information for the virtual carrier within the PBCH,which already carries the Master Information Block, or MIB in the hostcarrier centre band. As shown in FIG. 8, the MIB consists of 24 bits (3bits to indicate DL bandwidth, 8 bits to indicate the System FrameNumber or SFN, and 3 bits regarding the PHICH configuration). The MIBtherefore comprises 10 spare bits that can be used to carry locationinformation in respect of one or more virtual carriers. For example,FIG. 9 shows an example where the PBCH includes the MIB and locationinformation (“LI”) for pointing any virtual carrier terminal to avirtual carrier.

Alternatively, virtual carrier location information could be provided inthe centre band, outside of the PBCH. It can for example be alwaysprovided after and adjacent to the PBCH. By providing the locationinformation in the centre band but outside of the PBCH, the conventionalPBCH is not modified for the purpose of using virtual carriers, but avirtual carrier terminal can easily find the location information inorder to detect the virtual carrier, if any.

The virtual carrier location information, if provided, can be providedelsewhere in the host carrier, but it may be advantageous to provide itin the centre band, for example because a virtual carrier terminal mayconfigure its receiver to operate on the centre band and the virtualcarrier terminal then does not need to adjust its receiver settings forfinding the location information.

Depending on the amount of virtual carrier location informationprovided, the virtual carrier terminal can either adjust its receiver toreceive the virtual carrier transmissions, or it may require furtherlocation information before it can do so.

If for example, the virtual carrier terminal was provided with locationinformation indicating a virtual carrier presence and/or a virtualcarrier bandwidth but not indicating any details as to the exact virtualcarrier frequency range, or if the virtual carrier terminal was notprovided with any location information, the virtual carrier terminalcould then scan the host carrier for a virtual carrier (e.g. performinga so-called blind search process). Scanning the host carrier for avirtual carrier can be based on different approaches, some of which willbe presented below.

According to a first approach, a virtual carrier might only be insertedin certain pre-determined locations, as illustrated for example in FIG.10 for a four-location example. The virtual carrier terminal then scansthe four locations L1-L4 for any virtual carrier. If and when thevirtual carrier terminal detects a virtual carrier, it can then“camp-on” the virtual carrier to receive downlink data as describedabove. In this approach, the virtual carrier terminal may be providedwith the possible virtual carrier locations in advance, for example theymay be stored as a network-specific setting in an internal memory.Detection of a virtual carrier could be accomplished by seeking todecode a particular physical channel on the virtual carrier. Thesuccessful decoding of such a channel, indicated for example by asuccessful cyclic redundancy check (CRC) on decoded data, would indicatethe successful location of the virtual carrier

According to a second approach, the virtual carrier may include locationsignals such that a virtual carrier terminal scanning the host carriercan detect such signals to identify the presence of a virtual carrier.Examples of possible location signals are illustrated in FIGS. 11A to11D. In the examples of FIGS. 11A to 11C, the virtual carrier regularlysends an arbitrary location signal such that a terminal scanning afrequency range where the location signal is would detect this signal.An “arbitrary” signal is intended here to include any signal that doesnot carry any information as such, or is not meant to be interpreted,but merely includes a specific signal or pattern that a virtual carrierterminal can detect. This can for example be a series of positive bitsacross the entire location signal, an alternation of 0 and 1 across thelocation signal, or any other suitable arbitrary signal. It isnoteworthy that the location signal may be made of adjacent blocks ofresource elements or may be formed of non adjacent blocks. For example,it may be located at every other block of resource elements at the “top”(i.e. upper frequency limit) of the virtual carrier.

In the example of FIG. 11A, the location signal 353 extends across therange R₃₃₀ of the virtual carrier 330 and is always found at the sameposition in the virtual carrier within a sub-frame. If the virtualcarrier terminal knows where to look for a location signal in a virtualcarrier sub-frame, it can then simplify its scanning process by onlyscanning this position within a sub-frame for a location signal. FIG.11B shows a similar example where every sub-frame includes a locationsignal 354 comprising two parts: one at the top corner and one at thebottom corner of the virtual carrier sub-frame, at the end of thissub-frame. Such a location signal may be useful if, for example, thevirtual carrier terminal does not know the bandwidth of the virtualcarrier in advance as it can facilitate a clear detection of the top andbottom frequency edges of the virtual carrier band.

In the example of FIG. 11C, a location signal 355 is provided in a firstsub-frame SF1, but not in a second sub-frame SF2. The location signalcan for example be provided every two sub-frames. The frequency of thelocation signals can be chosen to adjust a balance between reducingscanning time and reducing overhead. In other words, the more often thelocation signal is provided, the less long it takes a terminal to detecta virtual carrier but the more overhead there is.

In the example of FIG. 11D, a location signal is provided where thislocation signal is not an arbitrary signal as in FIGS. 11A to 11C, butis a signal that includes information for virtual carrier terminals. Thevirtual carrier terminals can detect this signal when they scan for avirtual carrier and the signal may include information in respect of,for example, the virtual carrier bandwidth or any other virtualcarrier-related information (location or non-location information). Whendetecting this signal, the virtual carrier terminal can thereby detectthe presence and location of the virtual carrier. As shown in FIG. 11D,the location signal can, like an arbitrary location signal, be found atdifferent locations within the sub-frame, and the location may vary on aper-sub-frame basis.

Dynamic Variation of Control Region Size of Host Carrier

As explained above, in LTE the number of symbols that make up thecontrol region of a downlink sub-frame varies dynamically depending onthe quantity of control data that needs to be transmitted. Typically,this variation is between one and three symbols. As will be understoodwith reference to FIG. 5, variation in the width of the host carriercontrol region will cause a corresponding variance in the number ofsymbols available for the virtual carrier. For example, as can be seenin FIG. 5, when the control region is three symbols in length and thereare 14 symbols in the sub-frame, the virtual carrier is eleven symbolslong. However, if in the next sub-frame the control region of the hostcarrier were reduced to one symbol, there would be thirteen symbolsavailable for the virtual carrier in that sub-frame.

When a virtual carrier is inserted into a LTE host carrier, mobilecommunication terminals receiving data on the virtual carrier need to beable to determine the number of symbols in the control region of eachhost carrier sub-frame to determine the number of symbols in the virtualcarrier in that sub-frame if they are to be able to use all availablesymbols that are not used by the host carrier control region.

Conventionally, the number of symbols forming the control region issignalled in the first symbol of every sub-frame in the PCFICH. However,the PCFICH is typically distributed across the entire bandwidth of thedownlink LTE sub-frame and is therefore transmitted on sub-carrierswhich virtual carrier terminals capable only of receiving the virtualcarrier cannot receive. Accordingly, in one embodiment, any symbolsacross which the control region could possibly extend are predefined asnull symbols on the virtual carrier, i.e. the length of the virtualsub-carrier is set at (m-n) symbols, where m is the total number ofsymbols in a sub-frame and n is the maximum number of symbols of thecontrol region. Thus, resource elements are never allocated for downlinkdata transmission on the virtual carrier during the first n symbols ofany given sub-frame.

Although this embodiment is simple to implement it will be spectrallyinefficient because during sub-frames when the control region of thehost carrier has fewer than the maximum number of symbols, there will beunused symbols in the virtual carrier.

In another embodiment, the number of symbols in the control region ofthe host carrier is explicitly signalled in the virtual carrier itself.Once the number of symbols in the control region of the host carrier isknown, the number of symbols in the virtual carrier can be calculated bysubtracting the total number of symbols in the sub-frame from thisnumber.

In one example an explicit indication of the host carrier control regionsize is given by certain information bits in the virtual carrier controlregion. In other words an explicit signalling message is inserted at apredefined position in the virtual carrier control region 502. Thispredefined position is known by each terminal adapted to receive data onthe virtual carrier.

In another example, the virtual carrier includes a predefined signal,the location of which indicates the number of symbols in the controlregion of the host carriers. For example, a predefined signal could betransmitted on one of three predetermined blocks of resource elements.When a terminal receives the sub-frame it scans for the predefinedsignal. If the predefined signal is found in the first block of resourceelements this indicates that the control region of the host carriercomprises one symbol; if the predefined signal is found in the secondblock of resource elements this indicates that the control region of thehost carrier comprises two symbols and if the predefined signal is foundin the third block of resource elements this indicates that the controlregion of the host carrier comprises three symbols.

In another example, the virtual carrier terminal is arranged to firstattempt to decode the virtual carrier assuming that the control regionsize of the host carrier is one symbol. If this is not successful, thevirtual carrier terminal attempts to decode the virtual carrier assumingthat the control region size of the host carrier is two and so on, untilthe virtual carrier terminal successfully decodes the virtual carrier.

Downlink Virtual Carrier Reference Signals

As is known in the art, in OFDM-based transmission systems, such as LTE,a number of sub-carriers in symbols throughout the sub-frames aretypically reserved for the transmission of reference signals. Thereference signals are transmitted on sub-carriers distributed throughouta sub-frame across the channel bandwidth and across the OFDM symbols.The reference signals are arranged in a repeating pattern and can beused by a receiver to estimate the channel function applied to the datatransmitted on each sub-carrier using extrapolation and interpolationtechniques. These reference signals are also typically used foradditional purposes such as determining metrics for received signalpower indications, automatic frequency control metrics and automaticgain control metrics. In LTE the positions of the reference signalbearing sub-carriers within each sub-frame are pre-defined and aretherefore known at the receiver of each terminal.

In LTE downlink sub-frames, reference signals from each transmit antennaport are typically inserted on every sixth sub-carrier. Accordingly, ifa virtual carrier is inserted in an LTE downlink sub-frame, even if thevirtual carrier has a minimum bandwidth of one resource block (i.e.twelve sub-carriers) the virtual carrier will include at least somereference signal bearing sub-carriers.

There are sufficient reference signal bearing sub-carriers provided ineach sub-frame such that a receiver need not accurately receive everysingle reference signal to decode the data transmitted on the sub-frame.However, as will be understood the more reference signals that arereceived, the better a receiver will generally be able to estimate thechannel response, and hence fewer errors will typically be introducedinto the data decoded from the sub-frame. Accordingly, in order topreserve compatibility with LTE communication terminals receiving dataon the host carrier, in accordance with some examples of the presentinvention, the sub-carrier positions that would contain referencesignals in a conventional LTE sub-frame are retained in the virtualcarrier.

As will be understood, in accordance with examples of the presentinvention, terminals arranged to receive only the virtual carrierreceive a reduced number of sub-carriers compared to conventional LTEterminals which receive each sub-frame across the entire bandwidth ofthe sub-frame. As a result, the reduced capability terminals receivefewer reference signals over a narrower range of frequencies which mayresult in a less accurate channel estimation being generated.

In some examples a simplified virtual carrier terminal may have a lowermobility which requires fewer reference symbols to support channelestimation. However, in some examples of the present invention thedownlink virtual carrier includes additional reference signal bearingsub-carriers to enhance the accuracy of the channel estimation that thereduced capability terminals can generate.

In some examples the positions of the additional reference bearingsub-carriers are such that they are systematically interspersed withrespect to the positions of the conventional reference signal bearingsub-carriers thereby increasing the sampling frequency of the channelestimation when combined with the reference signals from the existingreference signal bearing sub-carriers. This allows an improved channelestimation of the channel to be generated by the reduced capabilityterminals across the bandwidth of the virtual carrier. In otherexamples, the positions of the additional reference bearing sub-carriersare such that they are systematically placed at the edge of thebandwidth of the virtual carrier thereby increasing the interpolationaccuracy of the virtual carrier channel estimates.

Alternative Virtual Carrier Arrangements

So far examples of the invention have been described generally in termsof a host carrier in which a single virtual carrier has been inserted asshown for example in FIG. 5. However, in some examples a host carriermay include more than one virtual carrier as shown for example in FIG.12. FIG. 12 shows an example in which two virtual carriers VC1 (330) andVC2 (331) are provided within a host carrier 320. In this example, thetwo virtual carriers change location within the host carrier bandaccording to a pseudo-random algorithm. However, in other examples, oneor both of the two virtual carriers may always be found in the samefrequency range within the host carrier frequency range and/or maychange position according to a different mechanism. In LTE, the numberof virtual carriers within a host carrier is only limited by the size ofthe host carrier. However, too many virtual carriers within the hostcarrier may unduly limit the bandwidth available for transmitting datato conventional LTE terminals and an operator may therefore decide on anumber of virtual carrier within a host carrier according to, forexample, a ratio of conventional LTE users/virtual carrier users.

In some examples the number of active virtual carriers can bedynamically adjusted such that it fits the current needs of conventionalLTE terminals and virtual carrier terminals. For example, if no virtualcarrier terminal is connected or if their access is to be intentionallylimited, the network can arrange to begin scheduling the transmission ofdata to LTE terminals within the sub-carriers previously reserved forthe virtual carrier. This process can be reversed if the number ofactive virtual carrier terminals begins to increase. In some examplesthe number of virtual carriers provided may be increased in response toan increase in the presence of virtual carrier terminals. For example ifthe number of virtual terminals present in a network or area of anetwork exceeds a threshold value, an additional virtual carrier isinserted in the host carrier. The network elements and/or networkoperator can thus activate or deactivate the virtual carriers wheneverappropriate.

The virtual carrier shown for example in FIG. 5 is 144 sub-carriers inbandwidth. However, in other examples a virtual carrier may be of anysize between twelve sub-carriers to 1188 sub-carriers (for a carrierwith a 1200 sub-carrier transmission bandwidth). Because in LTE thecentre band has a bandwidth of 72 sub-carriers, a virtual carrierterminal in an LTE environment preferentially has a receiver bandwidthof at least 72 sub-carriers (1.08 MHz) such that it can decode thecentre band 310, therefore a 72 sub-carrier virtual carrier may providea convenient implementation option. With a virtual carrier comprising 72sub-carriers, the virtual carrier terminal does not have to adjust thereceiver's bandwidth for camping on the virtual carrier which maytherefore reduce complexity of performing the camp-on process, but thereis no requirement to have the same bandwidth for the virtual carrier asfor the centre band and, as explained above, a virtual carrier based onLTE can be of any size between 12 to 1188 sub-carriers. For example, insome systems, a virtual carrier having a bandwidth of less than 72sub-carriers may be considered as a waste of the virtual carrierterminal's receiver resources, but from another point of view, it may beconsidered as reducing the impact of the virtual carrier on the hostcarrier by increasing the bandwidth available to conventional LTEterminals. The bandwidth of a virtual carrier can therefore be adjustedto achieve the desired balance between complexity, resource utilization,host carrier performance and requirements for virtual carrier terminals.

Uplink Transmission Frame

So far, the virtual carrier has been discussed primarily with referenceto the downlink, however in some examples a virtual carrier can also beinserted in the uplink.

In frequency division duplex (FDD) networks both the uplink and downlinkare active in all sub-frames, whereas in time division duplex (TDD)networks sub-frames can either be assigned to the uplink, to thedownlink, or further sub-divided into uplink and downlink portions.

In order to initiate a connection to a network, conventional LTEterminals make a random access request on the physical random accesschannel (PRACH). The PRACH is located in predetermined blocks ofresource elements in the uplink frame, the positions of which aresignaled to the LTE terminals in the system information signaled on thedownlink.

Additionally, when there is pending uplink data to be transmitted froman LTE terminal and the terminal does not already have any uplinkresources allocated to it, it can transmit a random access request PRACHto the base station. A decision is then made at the base station as towhich if any uplink resource is to be allocated to the terminal devicethat has made the request. Uplink resource allocations are then signaledto the LTE terminal on the physical downlink control channel (PDCCH)transmitted in the control region of the downlink sub-frame.

In LTE, transmissions from each terminal device are constrained tooccupy a set of contiguous resource blocks in a frame. For the physicaluplink shared channel (PUSCH) the uplink resource allocation grantreceived from the base station will indicate which set of resourceblocks to use for that transmission, where these resource blocks couldbe located anywhere within the channel bandwidth.

The first resources used by the LTE physical uplink control channel(PUCCH) are located at both the upper and lower edge of the channel,where each PUCCH transmission occupies one resource block. In the firsthalf of a sub-frame this resource block is located at one channel edge,and in the second half of a sub-frame this resource block is located atthe opposite channel edge. As more PUCCH resources are required,additional resource blocks are assigned in a sequential manner, movinginward from the channel edges. Since PUCCH signals are code divisionmultiplexed, an LTE uplink can accommodate multiple PUCCH transmissionsin the same resource block.

Virtual Uplink Carrier

In accordance with embodiments of the present invention, the virtualcarrier terminals described above can also be provided with a reducedcapability transmitter for transmitting uplink data. The virtual carrierterminals are arranged to transmit data across a reduced bandwidth. Theprovision of a reduced capability transmitter unit providescorresponding advantages to those achieved by providing a reducedcapability receiver unit with, for example, classes of devices that aremanufactured with a reduced capability for use with, for example, MTCtype applications.

In correspondence with the downlink virtual carrier, the virtual carrierterminals transmit uplink data across a reduced range of sub-carrierswithin a host carrier that has a greater bandwidth than that of thereduced bandwidth virtual carrier. This is shown in FIG. 13A. As can beseen from FIG. 13A, a group of sub-carriers in an uplink sub-frame forma virtual carrier 1301 within a host carrier 1302. Accordingly, thereduced bandwidth across which the virtual carrier terminals transmituplink data can be considered a virtual uplink carrier.

In order to implement the virtual uplink carrier, the base stationscheduler serving a virtual carrier ensures that all uplink resourceelements granted to virtual carrier terminals are sub-carriers that fallwithin the reduced bandwidth range of the reduced capability transmitterunits of the virtual carrier terminals. Correspondingly, the basestation scheduler serving the host carrier typically ensures that alluplink resource elements granted to host carrier terminals aresub-carriers that fall outside the set of sub-carriers occupied by thevirtual carrier terminals. However, if the schedulers for the virtualcarrier and the host carrier are implemented jointly, or have means toshare information, then the scheduler of the host carrier can assignresource elements from within the virtual carrier region to terminaldevices on the host carrier during sub-frames when the virtual carrierscheduler indicates that some or all of the virtual carrier resourceswill not be used by terminal devices on the virtual carrier.

If a virtual carrier uplink incorporates a physical channel that followsa similar structure and method of operation to the LTE PUCCH, whereresources for that physical channel are expected to be at the channeledges, for virtual carrier terminals these resources could be providedat the edges of the virtual carrier bandwidth and not at the edges ofthe host carrier. This is advantageous since it would ensure thatvirtual carrier uplink transmissions remain within the reduced virtualcarrier bandwidth.

Virtual Uplink Carrier Random Access

In accordance with conventional LTE techniques, it cannot be guaranteedthat the PRACH will be within the sub-carriers allocated to the virtualcarrier. In some embodiments therefore, the base station provides asecondary PRACH within the virtual uplink carrier, the location of whichcan be signaled to the virtual carrier terminals via system informationon the virtual carrier. This is shown for example in FIG. 13B in which aPRACH 1303 is located within the virtual carrier 1301. Thus, the virtualcarrier terminals send PRACH requests on the virtual carrier PRACHwithin the virtual uplink carrier. The position of the PRACH can besignaled to the virtual carrier terminals in a virtual carrier downlinksignaling channel, for example in system information on the virtualcarrier.

However, in other examples, the virtual carrier PRACH 1303 is situatedoutside of the virtual carrier as shown for example in FIG. 13C. Thisleaves more room within the virtual uplink carrier for the transmissionof data by the virtual carrier terminals. The position of the virtualcarrier PRACH is signaled to the virtual carrier terminals as before butin order to transmit a random access request, the virtual carrierterminals re-tune their transmitter units to the virtual carrier PRACHfrequency because it is outside of the virtual carrier. The transmitterunits are then re-tuned to the virtual carrier frequency when uplinkresource elements have been allocated.

In some examples where the virtual carrier terminals are capable oftransmitting on a PRACH outside of the virtual carrier, the position ofthe host carrier PRACH can be signaled to the virtual carrier terminals.The virtual carrier terminals can then simply use the conventional hostcarrier PRACH resource to send random access requests. This approach isadvantageous as fewer PRACH resources have to be allocated.

However, if the base station is receiving random access requests fromboth conventional LTE terminals and virtual carrier terminals on thesame PRACH resource, it is necessary that the base station is providedwith a mechanism for distinguishing between random access requests fromconventional LTE terminals and random access requests from virtualcarrier terminals.

Therefore, in some examples a time division allocation is implemented atthe base station whereby, for example, over a first set of sub-framesthe PRACH allocation is available to the virtual carrier terminals andover a second set of sub-frames the PRACH allocation is available toconventional LTE terminals. Accordingly, the base station can determinethat random access requests received during the first set of sub-framesoriginate from virtual carrier terminals and random access requestsreceived during the second set of sub-frames originate from conventionalLTE terminals.

In other examples, no mechanism is provided to prevent both virtualcarrier terminals and conventional LTE terminals transmitting randomaccess requests at the same time. However, the random access preamblesthat are conventionally used to transmit a random access request aredivided into two groups. The first group is used exclusively by virtualcarrier terminals and the second group is used exclusively byconventional LTE terminals. Accordingly, the base station can determinewhether a random request originated from a conventional LTE terminal ora virtual carrier terminal simply by ascertaining to what group therandom access preamble belongs.

Example Architecture

FIG. 14 provides a schematic diagram showing part of an adapted LTEmobile telecommunication system arranged in accordance with an exampleof the present invention. The system includes an adapted enhanced Node B(eNB) 1401 connected to a core network 1408 which communicates data to aplurality of conventional LTE terminals 1402 and reduced capabilityterminals 1403 within a coverage area (cell) 1404. Each of the reducedcapability terminals 1403 has a transceiver unit 1405 which includes areceiver unit capable of receiving data across a reduced bandwidth and atransmitter unit capable of transmitting data across a reduced bandwidthwhen compared with the capabilities of the transceiver units 1406included in the conventional LTE terminals 1402.

The adapted eNB 1401 is arranged to transmit downlink data using asub-frame structure that includes a virtual carrier as described withreference to FIG. 5 and to receive uplink data using a sub-framestructure as described with reference to FIG. 13B or 13C. The reducedcapability terminals 1403 are thus able to receive and transmit datausing the uplink and downlink virtual carriers as described above.

As has been explained above, because the reduced complexity terminals1403 receive and transmit data across a reduced bandwidth on the uplinkand downlink virtual carriers, the complexity, power consumption andcost of the transceiver unit 1405 needed to receive and decode downlinkdata and to encode and transmit uplink data is reduced compared to thetransceiver unit 1406 provided in the conventional LTE terminals.

When receiving downlink data from the core network 1408 to betransmitted to one of the terminals within the cell 1404, the adaptedeNB 1401 is arranged to determine if the data is bound for aconventional LTE terminal 1402 or a reduced capability terminal 1403.This can be achieved using any suitable technique. For example, databound for a reduced capability terminal 1403 may include a virtualcarrier flag indicating that the data must be transmitted on thedownlink virtual carrier. If the adapted eNB 1401 detects that downlinkdata is to be transmitted to a reduced capability terminal 1403, anadapted scheduling unit 1409 included in the adapted eNB 1401 ensuresthat the downlink data is transmitted to the reduced capability terminalin question on the downlink virtual. In another example the network isarranged so that the virtual carrier is logically independent of theeNB. More particularly the virtual carrier may be arranged to appear tothe core network as a distinct cell so that it is not known to the corenetwork that the virtual carrier has any relationship with the hostcarrier. Packets are simply routed to/from the virtual carrier just asthey would be for a conventional cell.

In another example, packet inspection is performed at a suitable pointwithin the network to route traffic to or from the appropriate carrier(i.e. the host carrier or the virtual carrier).

In yet another example, data from the core network to the eNB iscommunicated on a specific logical connection for a specific terminaldevice. The eNB is provided with information indicating which logicalconnection is associated with which terminal device. Information is alsoprovided at the eNB indicating which terminal devices are virtualcarrier terminals and which are conventional LTE terminals. Thisinformation could be derived from the fact that a virtual carrierterminal would initially have connected using virtual carrier resources.In other examples virtual carrier terminals are arranged to indicatetheir capability to the eNB during the connection procedure. Accordinglythe eNB can map data from the core network to a specific terminal devicebased on whether the terminal device is a virtual carrier terminal or anLTE terminal.

When scheduling resources for the transmission of uplink data, theadapted eNB 1401 is arranged to determine if the terminal to bescheduled resources is a reduced capability terminal 1403 or aconventional LTE terminal 1402. In some examples this is achieved byanalysing the random access request transmitted on the PRACH using thetechniques to distinguish between a virtual carrier random accessrequest and a conventional random access request as described above. Inany case, when it has been determined at the adapted eNB 1401 that arandom access request has been made by a reduced capability terminal1402, the adapted scheduler 1409 is arranged to ensure that any grantsof uplink resource elements are within the virtual uplink carrier.

In some examples, the virtual carrier inserted within the host carriercan be used to provide a logically distinct “network within a network”.In other words data being transmitted via the virtual carrier can betreated as logically and physically distinct from the data transmittedby the host carrier network. The virtual carrier can therefore be usedto implement a so-called dedicated messaging network (DMN) which is“laid over” a conventional network and used to communicate messagingdata to DMN devices (i.e. virtual carrier terminals).

Further Example Applications of Virtual Carriers

Having set out the concepts of virtual carriers of the kind described inco-pending UK patent applications numbered GB 1101970.0 [2], GB1101981.7[3], GB 1101966.8[4], GB 1101983.3[5], GB 1101853.8[6], GB1101982.5[7], GB 1101980.9[8] and GB 1101972.6 [9], some extensions ofthe virtual carrier concept in accordance with embodiments of theinvention are now described.

FIG. 15A is a schematic diagram representing how various regions in aLTE-type telecommunications network's time-frequency transmissionresource grid 1500 might be allocated for use to support a virtualcarrier such as described above. The extent of the resource grid 1500shown in FIG. 15A comprises 10 sub-frames 1512 (equivalent to one frameoverall) spaced along the horizontal time direction and spans abandwidth R₃₂₀ in frequency. Each sub-frame 1512 in FIG. 15A follows thesame general format as the sub-frame in FIG. 5 but is represented in amore simplified and schematic manner.

Thus, the transmission resource grid 1500 of FIG. 15A comprises hostcarrier PDCCH regions 1502, host carrier PDSCH regions 1506, virtualcarrier regions 1510 and reference symbol regions 1504. The virtualcarrier regions 1510 may comprise separate virtual carrier PDSCH regionsand virtual carrier PDCCH regions, such as schematically shown in FIG. 5by the separate regions identified by reference numerals 501 and 502.However, and as noted above, in other example implementations theprinciples of the virtual carrier operation might not mirror theseaspects of LTE-type networks. The reference symbol regions 1504 may beused solely for the host carrier, or these regions may also be receivedand used by terminals camped-on to the virtual carrier.

FIG. 15B is similar to and will be understood from FIG. 15A, but whereasFIG. 15A schematically represents regions of the time-frequencytransmission resource grid 1500 for both the host and virtual carriers,FIG. 15B schematically represents only regions associated with the hostcarrier (i.e. the host carrier PDCCH regions 1502, host carrier PDSCHregions 1506 and reference symbol regions 1504). In effect FIG. 15Brepresents what might be referred to as the host-carrier transmissionresource grid 1530. The regions of FIG. 15B shown without shading areassociated with the virtual carrier and do not “belong” to thehost-carrier transmission resource grid 1530.

FIG. 15C is also similar to and will be understood from FIG. 15A, butwhereas FIG. 15A schematically represents regions of the time-frequencytransmission resource grid 1500 for both the virtual and host carriers,FIG. 15C schematically represents only regions associated with thevirtual carrier (i.e. the virtual carrier regions 1510). FIG. 15C isthus the counter-part to FIG. 15B. In effect FIG. 15C represents whatmight be referred to as the virtual-carrier transmission resource grid1550. The regions of FIG. 15C shown without shading are associated withthe host carrier and do not “belong” to the virtual-carrier transmissionresource grid 1550.

The host-carrier transmission resource grid 1530 and virtual-carriertransmission resource grid 1550 complement one another in that one“fills” the spaces in the other so that when added together theycorrespond to the overall transmission resource grid 1510 of FIG. 15A.Thus to summarise some embodiments of the invention, communications aresupported using a plurality of Orthogonal Frequency Division Multiplex,OFDM, sub-carriers spanning a first frequency bandwidth (e.g. R₃₂₀ inFIGS. 15A to 15C). User-plane data may be communicated on the hostcarrier using a first group of the OFDM sub-carriers distributed acrossthe first frequency bandwidth (e.g. in regions 1506 of FIG. 15B) anduser-plane data may be communicated on the virtual carrier using asecond group of the OFDM sub-carriers distributed across a secondfrequency bandwidth, wherein the second frequency bandwidth is smallerthan and within the first frequency bandwidth (e.g. within regions 1510of FIG. 15C). However, control-plane data for the host carrier (e.g.PDCCH) may be communicated using both groups of the OFDM sub-carriers(e.g. in regions 1502 of FIG. 15B).

FIG. 16 schematically represents an architecture for a generallyLTE-based telecommunications system 1600 arranged in accordance with anembodiment of the invention. The system 1600 comprises an e-NodeB (basestation) 1610 arranged to communicate with a plurality of terminaldevices 1612 and a plurality of home eNode-Bs (home base stations) 1650in a serving cell of the base station/e-NodeB 1610. The home basestations 1650 are in turn arranged to communicate with respectiveterminal devices 1652 within their own serving cells 1658 (these servingcells being schematically represented as houses in FIG. 16). As isconventional, each of the base stations 1610, 1650 may comprise atransceiver unit 1610 a, 1650 a for transmission and reception ofwireless signals, and a controller unit 1610 b, 1650 b configured tocontrol the respective base stations 1610, 1650 to operate as desiredaccording to the principles described herein. For example, therespective controller units may comprise respective processor unitswhich are suitably configured/programmed to provide the desiredfunctionality using conventional programming/configuration techniquesfor base stations in wireless telecommunications systems

Many aspects of the general configuration represented in FIG. 16 followthe known principles of so-called micro-, femto- or pico-cells inwireless telecommunications systems. In accordance with these schemes, a“master” base station (base station 1610 in the example of FIG. 16)supports communications with conventional terminal devices within itscell range (such as terminal devices 1612 in FIG. 16). In addition, oneor more localised wireless telecommunication cells are supported bylocal base stations located within the cell footprint of the master basestation. These localised wireless telecommunication cells are oftenreferred to as micro-, femto- or pico-cells depending on the size of theoperating cell. They may also be referred to as “home” cells, reflectingan intended use as a hub for in-home wireless communications. For thesake of convenience the “local” base stations 1650 in this example willgenerally be referred to as femto base stations (femto e-NodeBs).However, it will be appreciated this is purely for the sake ofconvenience in terminology for distinguishing the different types ofbase station, and it is not to be interpreted as restricting embodimentsof the invention to any particular geographic cell size associated withthe “local” base stations 1650 of FIG. 16.

As well as the femto base stations 1650 supporting wirelesscommunication with terminal devices 1652 within their respective ranges,the femto base stations 1650 may also communicate with the master basestation 1610. The respective femto base stations 1650 may communicatewirelessly with the master base station 1600 and/or they may communicateover a wired link, for example via a conventional internet connection,such as may be provided by a DSL connection in the vicinity of therespective femto base stations 1650. Thus the femto base station(s) mayin effect provide a gateway/relay role for attached terminal devices1652 to communicate with the master base station 1650 and beyond (e.g.to the internet) via their associated femto base station 1650, or tocommunicate directly with the internet without involving the master basestation 1610.

The present inventors have recognized that a configuration of the kindshown in FIG. 16 involving terminal devices 1652 connected to a localfemto base station can be one in which there will be a desire to supportterminal devices having the characteristics of a machine typecommunication (MTC) device of the kind described above. Furthermore, inthese circumstances there will often be a desire to allow for the MTCdevices 1652 having relatively low capabilities (e.g. reduced operatingbandwidth) to be supported. By way of a concrete example, the extent ofa femto-cell 1658 such as shown in FIG. 16 might correspond to anindividual's home, and terminal devices 1652 within the user's homemight comprise, for example, a television receiver, a media player and agames console. These device may be configured wirelessly with oneanother and with devices outside the home via the femto base station1650.

The present inventors have recognized the virtual carrier conceptdescribed above could be implemented in the circumstances represented inFIG. 16 by the master base station 1610 supporting communications usingresources associated the host carrier (HC) and the femto base stations1650 supporting communications using resources associated the virtualcarrier(s) (VC). This approach is schematically represented in FIG. 16by the depiction of the host-carrier transmission resource grid 1530 ofFIG. 15B adjacent the “master” base station 1610 and the depiction ofthe virtual-carrier transmission resource grid 1550 of FIG. 15C adjacentrespective ones of the femto base stations 1650.

Transmission powers associated with the different femto base stations1650 and their associated terminal devices 1652 could be set at levelsthat results in little interference between signalling associated withthe different femto base stations 1650. This would allow the samevirtual carrier resources (e.g. such as those shown in FIG. 15C) to beused for communications associated with different femto base stations1650. For example, if the characteristic femto cell size is on the orderof tens of meters, and neighbouring femto cells are a comparabledistance apart, transmission powers that decay to non-reliable detectionover a corresponding characteristic distance could be used. Thisapproach can be transmission resource efficient because resourcesassociated with the virtual carrier can be re-used within differentfemto cells within the overall footprint of the same master basestation.

Thus, a mode of operating a telecommunications network in accordancewith an embodiment of the invention is one in which a virtual carrierand a host carrier are supported by different cell sites. A master (or“host”) base station can support communications with terminal deviceswhich are out of range of the femto base stations, and in some examplesalso communications with the femto base stations themselves, usingnon-virtual carrier resources. Transmissions from the host base stationmay maintain PDCCH and Reference Signal transmission on frequencies thatoverlap with the virtual carrier to preserve backward compatibility inthe manner described above. The femto base station(s), on the otherhand, use the virtual carrier resources to service device, such as MTCdevices, that are within the corresponding femto cell(s). By followingthis approach, the telecommunications system can support an increasednumber of low capability terminal devices on the virtual carrier byresource re-use at different geographic locations, while maintainingbackward compatibility with what might be termed legacy devicesoperating according to an existing telecommunications standard.

In the example virtual carrier implementation described above withreference to FIGS. 15A to 15C and FIG. 16, reference symbols 1504associated with the host carrier are maintained across the fulloperating bandwidth of the host carrier. This results in “holes” in thevirtual carrier transmission resource grid 1550 of FIG. 15C. In anotherexample, the host carrier may be configured to not use some of thesereference symbols to increase the available resources/reduce disruptionon the virtual carrier. For example, the host carrier may be configuredto not use reference symbols on OFDM sub-carriers that are within thegroup of OFDM sub-carriers comprising the bandwidth of the virtualcarrier and outside the host-carrier PDCCH. This approach isschematically represented in FIG. 17A.

FIG. 17A schematically shows a host-carrier transmission resource grid1710 which is similar to and will be understood from the host-carriertransmission resource grid 1530 of FIG. 15B, with corresponding elementsbeing identified by corresponding reference numerals. However, thehost-carrier transmission resource grid 1710 of FIG. 17A differs fromthe host-carrier transmission resource grid 1530 of FIG. 15B in thearrangement of reference symbols 1504. In particular, in thehost-carrier transmission resource grid 1710 of FIG. 17A there are nohost-carrier reference symbols transmitted on the OFDM sub-carriersmaking up the virtual carrier on symbols which are outside thehost-carrier PDCCH. That is to say, there are no “holes” in the virtualcarrier resources in each sub-frame (i.e. in the parts without shadingin FIG. 17A) which are associated with reference symbols 1504 for thehost carrier.

With the host transmission resource grid represented in FIG. 17A, aconventional LTE-compliant terminal device (a “legacy” device) camped-onto the host carrier will find that a number of the expected referencesymbols are not received. In accordance with the defined LTE standards,a legacy device will in these circumstances assume it has for somereason missed the reception of these reference signals, and willinterpolate channel conditions over the virtual carrier frequencies fromthe reference signals it has received, i.e. those transmitted outsidethe virtual carrier bandwidth in FIG. 17A. (The arrangement of referencesymbol within the OFDM symbols associated with the host's PDCCH 1502remains unaffected in this example.) The need for the legacy terminal tointerpolate channel conditions across the virtual channel frequencieswill to some extent degrade a legacy device's channel estimations forthe virtual carrier frequencies. However, a legacy device camped-on tothe host carrier will not generally be serviced in the virtual carrierfrequency band (apart from in respect of PDCCH for which host referencesymbols are maintained in this example). Accordingly, the impact of thelegacy devices interpolating channel conditions for these regions of thespectrum may be minimal.

FIG. 17B schematically shows a host-carrier transmission resource grid1720 which is similar to and will be understood from the host-carriertransmission resource grids 1530, 1710 of FIGS. 15B and 17A, withcorresponding elements being identified by corresponding referencenumerals. However, the host-carrier transmission resource grid 1720 ofFIG. 17B differs from the host-carrier transmission resource grids 1530,1710 of FIGS. 15B and 17A in the arrangement of reference symbols 1504.In particular, the host-carrier transmission resource grid 1720 of FIG.17B represents a compromise between that of FIG. 15B (with full use ofhost reference symbols within virtual carrier resources) and that ofFIG. 17A (with no use of host reference symbols within virtual carrierresources). Thus in the example of FIG. 17B there is a reduced densityof host-carrier reference symbols transmitted on the OFDM sub-carriersmaking up the virtual carrier on symbols which are outside thehost-carrier PDCCH as compared to elsewhere in the sub-frame. That is tosay, there are fewer “holes” in the virtual carrier resources (i.e. inthe parts without shading in FIG. 17B) associated with reference symbols1504 for the host carrier as compared to FIG. 15B, but more holes ascompared with FIG. 17A. This provides for a balance between improvedchannel estimation on the host carrier and reduced disruption of thevirtual carrier by host-carrier reference symbols. In the example shownin FIG. 17B the host carrier reference symbols that are transmitted atthe times and frequencies which are primarily reserved for the virtualcarrier are transmitted near to the boundary between the virtual carrierand host carrier. This can help reduce interpolation effects in the hostcarrier region.

A consideration for some example implementations in which a host carrierand a virtual carrier are transmitted from different cell sites is timeand frequency matching between the master base station and respectivefemto base stations. It is expected for optimum operation that thevirtual carrier resources associated with the femto base station shouldmap closely to the corresponding “gaps” in the host carrier resourcesassociated with the master base station, and vice versa. That is to say,improved operation may be achieved if the schematic grids in FIGS. 15Band 15C add together to form the grid represented in FIG. 15A withappropriate registration and without undue offset. This will helpminimise interference between the host and virtual carriers.

In terms of temporal synchronisation (i.e. corresponding left-rightalignment between FIGS. 15B and 15C) the femto base station(s) cansimply rely on conventional synchronisation signalling from the masterbase station transmitted on the host carrier (e.g. PSS and SSS) to ineffect lock the femto base station(s) clock to the master base stationclock.

In terms of frequency synchronisation/matching (i.e. corresponding toup-down alignment between FIGS. 15B and 15C) it would be possible tomatch frequencies to an appropriate degree with any conventionaltechniques for mutual-locking of separated oscillators, or by simplyusing sufficiently high quality frequency standards at each geographiclocation. However, another approach would be to introduce guard regionsat the frequency interface(s) between OFDM sub-carriers associated withthe host carrier and OFDM sub-carriers associated with the virtualcarrier in the relevant symbols (i.e. the symbols during which bothcarriers are potentially active). This approach is schematicallyrepresented in FIG. 18.

FIG. 18 schematically shows a transmission resource grid 1810 which issimilar to and will be understood from the transmission resource grid1510 of FIG. 15A, with corresponding elements being identified bycorresponding reference numerals. As with FIG. 15A, FIG. 18schematically shows host carrier PDCCH regions 1502, host carrier PDSCHregions 1506, virtual carrier regions 1510 and reference symbol regions1504. However, the transmission resource grid 1810 of FIG. 18 differsfrom that of FIG. 15A by the inclusion of guard regions 1820 (showncross hatched in the figure).

The guard regions in this example comprise a group of OFDM sub-carrierslocated between (in frequency space) the host carrier PDSCH regions 1506and the virtual carrier regions 1510 during symbols when the virtualcarrier is being used. The guard regions do not extend into the regionscorresponding to PDCCH 1502 on the host carrier. This is to helpmaximise compatibility with legacy terminal devices using the hostcarrier (there should be no risk of interference from frequencydifferences in this region in this example implementation because thevirtual carrier is not allocated for use at these times).

In accordance with some example embodiments of the invention neither themaster base station nor the femto base station(s) allocate transmissionresources within the guard regions for use. Thus if the virtual carrierresource grid drifts in frequency relative to the host carrier resourcegrid, and the magnitude of the drift is within the frequency extent ofthe guard regions, there should be no significant increase ininterference between the two carriers. Thus the extent of the guardregions in frequency may be chosen in dependence on the expectedfrequency drifts that might arise between the master base station andfemto base station(s). If small frequency drifts are expected,correspondingly small guard regions may be used, for example spanningonly a single or small number of OFDM sub-carriers. If larger frequencydrifts are expected, correspondingly larger guard regions may be definedto help reduce the potential impact of overlapping transmissions.

In cases where reference symbols associated with the host carrier aremaintained on OFDM sub-carriers within the operating frequency extent ofthe virtual carrier, such as schematically shown in FIGS. 15A, 17B and18, it may also be helpful to include guard regions which are adjacentto (in terms of frequency) the reference symbols in the virtual carrierbandwidth. This approach is schematically represented in FIG. 19.

FIG. 19 schematically shows a portion of a transmission resource gridwhich is similar to and will be understood from (a portion of) thetransmission resource grid 1510 of FIG. 15A, with corresponding elementsbeing identified by corresponding reference numerals. The portion of thegrid shown in FIG. 19 corresponds to the bandwidth of the virtualcarrier for one sub-frame. Thus FIG. 18 schematically shows a portion ofthe host carrier PDCCH region 1502, a virtual carrier region 1510, andreference symbols 1504 associated with the host carrier. However, theportion of transmission resource grid of FIG. 18 differs fromcorresponding portions of the transmission resource grid 1510 of FIG.15A by the inclusion of reference symbol guard regions 1920 (shown crosshatched in the figure). These may, for example, be provided in additionto the carrier-interface guard regions 1820 shown in FIG. 18.

The reference-symbol guard regions 1920 in this example comprise a groupof OFDM sub-carriers located adjacent to (in frequency space) hostcarrier reference symbols occurring within the virtual carrier region1510 during OFDM symbols when these reference symbols are beingtransmitted. In this example the reference-symbol guard regions 1920 areprovided on both the upper and lower frequency sides of each referencesymbol 1504.

In accordance with some example embodiments of the invention neither themaster base station nor the femto base station(s) allocate transmissionresources within the reference symbol guard regions for use. Thus, ifthe virtual carrier resource grid drifts in frequency relative to thehost carrier resource grid, and the magnitude of the drift is within thefrequency extent of the reference symbol guard regions, there should beno significant increase in interference between the host carrierreference symbols and transmissions on the virtual carrier(s).

While in some examples the different guard regions 1820, 1920 may not beused at all for data transmissions, in other examples these regions maybe used by one or both of the master and femto base stations to transmitdata with high error resistance (e.g. low rates), so that even if thereis increased interference due to non-ideal frequency registrationbetween the host and virtual carriers, some data transmitted in theguard regions can nonetheless be decoded with a reasonable chance ofsuccess.

It will be appreciated that various modifications can be made to theembodiments described above without departing from the scope of thepresent invention as defined in the appended claims. In particularalthough embodiments of the invention have been described with referenceto an LTE mobile radio network, it will be appreciated that the presentinvention can be applied to other forms of network such as GSM, 3G/UMTS,CDMA2000, etc. The term MTC terminal as used herein can be replaced withuser equipment (UE), mobile communications device, terminal device etc.Furthermore, although the term base station has been usedinterchangeably with e-nodeB it should be understood that there is nodifference in functionality between these network entities.

It will also be appreciated that although the above description of ascheme in which host carriers and virtual carriers are supported bygeographically separated base stations has primarily focussed ondownlink transmissions by way of example, the same concepts can equallybe applied for uplink transmissions.

Thus, there has been described a method and apparatus for communicatingdata in a wireless telecommunications system between a plurality of basestations and a plurality of terminal devices using a plurality ofOrthogonal Frequency Division Multiplex, OFDM, sub-carriers spanning afirst frequency bandwidth. The method comprises communicating databetween a first base station and a first terminal device using a firstgroup of the OFDM sub-carriers distributed across the first frequencybandwidth, communicating data between a second base station, which isgeographically separated from the first base station, and a secondterminal device using a second group of the OFDM sub-carriersdistributed across a second frequency bandwidth and not using the firstgroup of sub-carriers, wherein the second frequency bandwidth is smallerthan and within the first frequency bandwidth, and the first and secondgroups are mutually exclusive; and communicating control-plane databetween the first base station and the first terminal device using acombination of the first and second groups of the OFDM sub-carriers.Guard regions may be provided in the frequency domain betweenfrequencies of transmissions associated with the first base station andfrequencies of transmissions associated with the second base station.This can help reduce unintended overlaps in transmissions from the twobase stations if there is a frequency mismatch between them.

Further particular and preferred aspects of the present invention areset out in the accompanying independent and dependent claims. It will beappreciated that features of the dependent claims may be combined withfeatures of the independent claims in combinations other than thoseexplicitly set out in the claims.

The invention claimed is:
 1. A method of operating a base station forcommunicating data between the base station and a plurality of terminaldevices in a wireless telecommunications system using a plurality ofOrthogonal Frequency Division Multiplex, OFDM, sub-carriers spanning afirst frequency bandwidth, the method comprising: communicating networkuser traffic using a first group of the OFDM sub-carriers distributedacross the first frequency bandwidth; communicating no network usertraffic using a second group of the OFDM sub-carriers distributed acrossa second frequency bandwidth, wherein the second frequency bandwidth issmaller than and within the first frequency bandwidth; and communicatingcontrol signaling traffic with the plurality of terminal devices using acombination of the first and second groups of the OFDM sub-carriers. 2.The method of claim 1, wherein a lower amount of network user traffic iscommunicated using OFDM sub-carriers arranged in frequency at a boundarybetween OFDM sub-carriers in the first group and OFDM sub-carriers inthe second group as compared to an amount of network user trafficcommunicated using OFDM sub-carriers away from this boundary.
 3. Themethod of claim 2, wherein network user traffic is communicated at areduced rate using OFDM sub-carriers at the boundary.
 4. The method ofclaim 2, wherein no network user traffic is communicated using OFDMsub-carriers at the boundary.
 5. The method of claim 1, furthercomprising: transmitting reference signals using both the first andsecond groups of the OFDM sub-carriers.
 6. The method of claim 5,wherein a density of the reference signals in the time and/or frequencydomain transmitted from the base station using the first group of theOFDM sub-carriers is greater than a density of reference signals in thetime and/or frequency domain transmitted from the base station using thesecond group of the OFDM sub-carriers.
 7. The method of claim 1, furthercomprising: transmitting reference signals from the base station usingonly the first group of the OFDM sub-carriers at times when controlsignaling traffic is not communicated by the base station.
 8. The methodof claim 1, wherein the first frequency bandwidth corresponds to a hostcarrier, and the second frequency bandwidth corresponds to a virtualcarrier embedded within the host carrier.
 9. A base station forcommunicating data with a plurality of terminal devices in a wirelesstelecommunications system using a plurality of Orthogonal FrequencyDivision Multiplex, OFDM, sub-carriers spanning a first frequencybandwidth, wherein the base station is configured to: communicatenetwork user traffic with the plurality of terminal devices using afirst group of the OFDM sub-carriers distributed across the firstfrequency bandwidth; communicate no network user traffic with theplurality of terminal devices using a second group of the OFDMsub-carriers distributed across a second frequency bandwidth, whereinthe second frequency bandwidth is smaller than and within the firstfrequency bandwidth; and communicate control signaling traffic with theplurality of terminal devices using a combination of the first andsecond groups of the OFDM sub-carriers.
 10. The base station of claim 9,wherein the base station is configured such that a lower amount ofnetwork user traffic is communicated using OFDM sub-carriers arranged infrequency at a boundary between OFDM sub-carriers in the first group andOFDM sub-carriers in the second group as compared to the amount ofnetwork user traffic communicated using OFDM sub-carriers away from theboundary.
 11. The base station system of claim 8, wherein the basestation is configured such that network user traffic is communicated ata reduced rate using OFDM sub-carriers at the boundary.
 12. The basestation of claim 8, wherein the base station is configured such that nonetwork user traffic is communicated using OFDM sub-carriers at theboundary.
 13. The base station of claim 9, wherein the base station isconfigured to transmit reference signals using both the first and secondgroups of the OFDM sub-carriers.
 14. The base station of claim 13,wherein the base station is configured such that the transmitted densityof reference signals in time and/or frequency on the first group of theOFDM sub-carriers is greater than on the second group of the OFDMsub-carriers.
 15. The base station of claim 9, wherein the base stationis configured to transmit reference signals using only the first groupof the OFDM sub-carriers at times when control signaling traffic is notbeing communicated by the base station.
 16. A method of operating aterminal device for communicating data in a wireless telecommunicationssystem for communicating data between a plurality of base stations and aplurality of terminal devices using a plurality of Orthogonal FrequencyDivision Multiplex, OFDM, sub-carriers spanning a first frequencybandwidth, the wireless telecommunications system comprising a macrobase station configured to communicate using a first group of the OFDMsub-carriers distributed across the first frequency bandwidth; and ahome base station configured to communicate using a second group of theOFDM sub-carriers distributed across a second frequency bandwidth,wherein the second frequency bandwidth is smaller than and within thefirst frequency bandwidth, and wherein the terminal device is configuredto communicate only with the home base station using the second group ofthe OFDM sub-carriers and is not operable to communicate with the firstbase station even when the terminal device is in a coverage area of boththe macro base station and the home base station.
 17. A terminal devicefor use in a wireless telecommunications system for communicating databetween a plurality of base stations and a plurality of terminal devicesusing a plurality of Orthogonal Frequency Division Multiplex, OFDM,sub-carriers spanning a first frequency bandwidth, the wirelesstelecommunications system comprising a macro base station configured tocommunicate using a first group of the OFDM sub-carriers distributedacross the first frequency bandwidth; and a home base station configuredto communicate using a second group of the OFDM sub-carriers distributedacross a second frequency bandwidth, wherein the second frequencybandwidth is smaller than and within the first frequency bandwidth, andwherein the terminal device is configured to communicate only with thehome base station using the second group of the OFDM sub-carriers and isnot operable to communicate with the macro base station even when theterminal device is in a coverage area of both the macro base station andthe home base station.